1. Technical Field
The present disclosure relates to an image sensor including at least two photosites separated by an isolation region, and to a method for forming the isolation region between the photosites.
2. Description of the Related Art
Camera modules typically include a lens, an infrared filter, and an image sensor in the form of a semiconductor chip comprising several photosites, one or more photosites corresponding to a pixel of a captured image. The photosites are generally arranged in a matrix and may include photodiodes, photogates, or photoconductors, etc. In general, above the photosites are distributed a mosaic of microlenses to converge incident light rays on the photosites and a mosaic of colored filters to filter the light. Layers of dielectric materials are also generally present above the photosites, and serve to insulate conductive lines interconnecting the sites, and act as passivation layers against humidity and impurities. Channels are formed in the dielectric layers through which the light rays must pass to reach the photosites.
An image sensor uses the “photoelectric effect” to transform light information into an electrical signal. A photon of an incident light ray penetrates into the material of the photosite, and if the energy of the photon is equal to or greater than the material's energy bandgap, an “electron-hole pair” is generated. The number of electrons per photosite may be measured and used to supply light information for the image, and the number of electrons read per number of incident photons is known as the quantum efficiency. Photons with short wavelengths have a shallow penetration depth, such as 0.4 μm for wavelengths of 450 nm (blue light), and photons with long wavelengths have a deeper penetration depth, such as 2.4 μm for wavelengths of 600 nm (red light). As adjacent photosites correspond to differently-filtered light (for example red, green, or blue) as well as to different image pixels, their information should be independently provided. The angular response of a pixel is the light intensity measured vs.
the angle of the incident light. For example, for a “field of view” from −45° to 45°, with 0° being perpendicular to the surface of the photosite, the intensity is generally the highest at 0°, and decreases towards the edges of the field, in the shape of a bell curve. Ideally, the light intensity would be the same for the entire field of view, but depending upon its incident angle, a light ray intended for one photosite may cross to another photosite, causing optical crosstalk. The sensitivity of an image sensor thus depends on quantum efficiency and the amount of crosstalk that occurs.
U.S. Pat. No. 7,279,770 discloses an image sensor comprising a deep trench isolation (DTI) of at least 4 μm between adjacent photosites. The depth of the trench therefore prevents deeply-generated electrons from passing under the isolation region from one photosite to another. Furthermore, the trench is filled with a conductive material such as polysilicon, after having been covered by a liner, such as a dielectric layer. The conductive material may be more easily deposited into deep trenches as opposed to conventional isolation materials such as oxide which may cause undesirable voids or air gaps. The liner prevents generated electrons from passing through the trench from one photosite to another, which is known as electrical crosstalk.
FIG. 1 is a cross-sectional view of a part of a conventional image sensor IS1 according to the teaching of the above-mentioned patent. The image sensor IS1 includes, from bottom to top, a substrate 10 comprising three photosites S1, S2, S3; an insulator layer 70; a stack 80 of dielectric layers 80-1 to 80-N, each layer comprising conductive lines 90-1 to 90-N to extract electrical information from and power the photosites; colored filters F1, F2, F3; and microlenses L1, L2, L3. Isolation regions 11, 12 extend between sites S1, S2 and S2, S3 respectively, and each includes a deep trench made in the substrate 10 and having its bottom and sidewalls covered by a thin liner 20, and filled with a conductive material 30.
FIG. 1 schematically shows three incident light rays R1, R2, R3. Light ray R1 passes through filter F1, penetrates into photosite S1, and generates an electron E1 read by photosite S1. Light ray R2 passes through filter F2 and has an angle and a penetration depth such that it enters the isolation region I1 instead of a photosite. The photons of ray R2 are absorbed in the conductive material 30 of the isolation region I1, and a trapped electron E2 is generated. The quantum efficiency is thus decreased as the photon of ray R2 is not captured by a photosite. Light ray R3 passes through filter F3, but has an angle such that instead of being absorbed in the corresponding photosite S3, it crosses the isolation region I2 and penetrates into the adjacent photosite S2, where it is absorbed and generates an electron E3. Electron E3 is thus incorrectly read by photosite S2 instead of by photosite S3, and optical crosstalk occurs.